1. Field of the Invention
The present invention relates to an apparatus for transferring thin workpieces such as semiconductor wafers (hereinafter referred to as wafers). The apparatus can be used in a system for manufacturing such wafers. The invention particularly relates to a wafer transfer apparatus having a wafer transfer portion which is capable of supporting a wafer storing case, for carrying and temporarily storing wafers.
2. Description of the Prior Art
An apparatus of interest is disclosed in Japanese Utility Model Laying-Open Gazette No. 128731/1988. FIG. 1 is a perspective view of a wafer transfer portion of the apparatus described in this prior art.
The wafer transfer portion 51 to which wafers are fed includes a support frame 54, a bottom plate 8, spacers 55 and a top plate 9. The bottom plate 8 is fixed on the support frame 54 and includes three support protrusions 17 nearly at its center for placing a wafer 2 thereon. The spacers 55 are fixed on the upper surface of the bottom plate 8. The top plate 9 is coupled to the bottom plate 8 in an openable and closable manner as illustrated in FIG. 1 by the line B through the spacers 55. The bottom plate 8, the spacers 55 and the top plate 9 define a space through which wafers 2 can pass therethrough.
The wafer transfer portion 51 is supported and moved up and down by an elevating means. The elevating means includes a drive screw 61 and a pair of support rods 62. The support rods 62 pass through holes located in the support frame 54. The drive screw 61 is engaged with a threaded hole in the support frame 54. The drive screw 61 is rotated by a drive motor (not illustrated in FIG. 1) whereby the wafer transfer portion 51 is elevated or lowered.
Wafers are transported into and from the wafer transfer portion 51 by wafer feed means and wafer discharge means (not illustrated in FIG. 1).
A wafer cassette can be mounted upon the top plate 9. The wafer cassette moves up and down together with the wafer transfer portion 51.
Referring to FIGS. 2(a)-(d), to transfer wafers, the wafer transfer portion 51 is positioned at a predetermined level by the elevating means. As illustrated in FIG. 2(a), a wafer 2 is fed into the wafer transfer portion 51 by wafer feed means 66a. Referring to FIG. 2(b), the wafer transfer portion 51 is raised by the elevating means. The support protrusions 17 move upward and the respective top ends thereof support and raise the wafer 2. Then, the wafer feed means 66a returns outside the wafer transfer portion 51.
Referring to FIG. 2(c), wafer discharge means 66b moves into the wafer transfer portion 51.
Referring to FIG. 2(d), when a leading end of the wafer discharge means 66b comes near a position under the center of the wafer 2, the wafer transfer portion 51 is lowered by the elevating means. Then, the protrusions 17 lower so that the wafer discharge means 66a receives the wafer 2. After that, the wafer discharge means 66b carries the wafer 2 outside the wafer transfer portion 51 and the transfer of the wafer 2 is completed.
Now, the method of storing wafers 2 in a wafer cassette will be described. The wafer transfer portion 51 is lowered by the elevating means, so that the wafer cassette is positioned at a predetermined level. Then, the wafer 2 is placed in the wafer cassette by the wafer feed means 66a.
To take the wafers 2 out of the wafer cassette, the wafer transfer portion 51 is lowered by the elevating means until the wafer cassette is positioned at a predetermined level. Then, the wafer feed means 66a takes out one of the wafers 2 from the wafer cassette. The wafer transfer portion 51 is then positioned at a predetermined level for normal transfer of wafers and then wafers are transferred normally. If there are still wafers to be taken out from the cassette, the above described operation is repeated.
The above described convention apparatus uses a mechanical means for transporting wafers, which does not have the problems of dust or the like which occur when wafers are transported by air currents.
One of the disadvantages with the conventional apparatus is that it is not easy to automate wafer cassette replacement. This is because the wafer cassette is placed on the wafer transfer portion and moves upward and downward together with the wafer transfer portion as wafers are transferred.
Another disadvantage is that the apparatus' elevating and lowering mechanism is complicated. The elevations and lowering mechanism is complicated because it must perform each of the various tasks of feeding wafers to the wafer transfer portion, storing wafers in the cassette, and taking out wafers from the cassette.